Method of treating natural gas



`lune 19,l 1951 w. w. BQDLE ErAL 'METHOD oF TREATING NATURAL GAS Filed Nov. l2, 1946 Gttornegs Patented June 19, 1951 UNITED STATES PATENT OFFICE METHOD F TREATING NATURAL GAS William W. Bodle, North Kansas City, M0., and

Walter W. Deschner, Kansas City, Kans., assigners to J. F. Pritchard & Co., Kansas City, Mo., a corporation of Missouri Application November 12, 1946, Serial No. 709,308

Claims. (Cl. 62-1'155)` This invention relates Lo a method lor removing inert constituents, particularly nitrogen, from natural gas, and has for its principal object to provide an economical method for removing nitrogen by low temperature liquefaction and fractionation of the natural gas and thereby providing a product relatively free of nitrogen and a waste gas relatively free of hydrocarbons.

Other objects of the invention are to provide for the low temperatures necessary to liquefy the gas through heat exchange of the entra-ined gas with processed factions which carry low temperatures effected through expansion in low temperature expansion engines, and to utilize as far as practicable power of the expansion engines for operating equipment required in processing of the gas.

In accomplishing these and other objects of the invention. we have provided an improved process and apparatusior practicing the process illustrated in the accompanyingdrawingwherein:

Fig. 1 is a flow diagram of a form of the invention for removing the nitrogen in suiiicient amount from natural gas to provide a product gas of a certain B. t. u. value. y

Fig. 2 is a similar flow diagram of an apparatus Wherewith substantially the entire nitrogen content of the natural gas may be removed.

Referring more in detail to Fig. l of the drawing:

l designates a pipe line through which the natural gas to be processed is transported from a source of supply to a processingT apparatus embodying the present invention. The gas processinf.I apparatus includes a carbon dioxide and hydrogen sulfide removal unit designated 2 and a dehydrating unit 3 through which the natural gas is conducted preparatory for liquefaction of the gas.

The gas is conducted from the dehydrator l through a pipe 4 leading to a heat exchanger 5 wherein it is cooled to low temperature by heat exchange with effluent streams of low temperature gas fractions leaving the treating apparatus through passages 6, 1, 8 and 9 arranged in heat exchange relation with the passageway I0 through which the incoming gas passes, the gas being cooled to a temperature that a phase separation into liquid and vapor takes place in the feed.

Attention is here directed to the fact that the temperatures and pressures carried on in the process are dependent upon the chemical composition of the gas, and consequently, are variable and it is to be understood that the temperatures and pressures specied are illustrative for one type of natural gas to facilitate better understanding of the process. Therefore, it will be conper cubic foot higher heating value at 60 F. A

' when the treated gas is mixed with the, untreated standard cubic foot of natural gas is one cubic foot of natural gasat 14.7 pounds per square inch absolute at F. It may also be assumed that 50,000,000 s. c. fad. product gas is to be delivered from the system at 615 p. s. i. a. having a-higher heat value at 60 F. of 1055 B. t. u. per cubic foot, and which contains 9.6 per cent nitrogen. Since it is desired to raise the heating value, it is necessary to treat only that portion of the flow so that portion a product gas of the desired B. t. u. value will result. To accomplish the result, the portion of the untreated gas may be figured at 17,836,000 s. c. f. d., which portion of the now is diverted from the pipe 4 through a pipe il to a compression unit indicated at l2 whereby gas is raised to the delivery pressure and discharged into the delivery pipe line I3 lfor mixture with treated gas to give a product gas having the above specifications. The remaining portions of the ow -or 38,800,000 s. c. f. d. at a temperature of approximately F. is to be treated and is delivered to the passageway I0 oi the heat exchanger 5. The gas is cooled in the heat exchanger to approximately 200 F. while the pressure is maintained at approximately 200 p. s. i. a.

The highly cooled gas flow passes on through a pipe I4 into a separator l5 wherein it separates into a liquid phase containing '7.3 per cent nitrogen and a gas phase containing 41.7 per cent nitrogen. The liquid phase drops to the bottom of the separator and is carried off through a pipe i6 leading to the heat exchanger passage 'l Where it gives up its cold by countercurrent heat exchange with the incoming gas traveling through the passage I0 so that it becomes a gas 10W in nitrogen and which is usable in making up the product gas. Therefore, the gas is discharged from the heat exchanger and conducted through a pipe Il leading to compressionv units indicated at i8 for delivery to the pipe line i3 at the required pressure to mix with the untreated portion of the gas flow.

The gas carrying the major portion of nitrogenV changers 20, 2| and'22 at a pressure of 200 p. s. i. a.

s; c. f. d. (standard cubic tionating tower.

exchanger the nitrogen has considerable pressure rand 'a `temperature of 200 F. Upon flow if'through the heat exchangers the gas isliqueed and fed through a pipe 23 under control of a throttle valve 24 into the top of a fractionating tower 25 at a temperature of -292 F. and pressure of 50 p. s'. i. a.

The remainder of the gas in vapor phase is diverted through a pipe 26 and is delivered to an expansion engine 21. In passing through the expansion engine 21, the gas is cooled essentially to its liquefaction temperature and is delivered to the bottom of the Iractionator 25 at a temperature of -258 F. and a pressure of 50 p. s. i. a. If desired, a portion of the discharge from the expansion engine 21 may be taken 01T through a pipe 28 for flow through the passageway 8 of the heat exchanger to assist in maintaining the desired low temperature of the incoming gas. After absorbing heat in the passageway 8, the gas is discharged through a pipe 29 and may be used as fuel gas to operate the compressors.

The fluid passing into the top of the fractionator tower from the heat exchangers 20, 2| and 22 assumes a temperature of 292 F. and enters the top of the fractionator at a pressure of 50 p. s. i. a. and passes downwardly through the tower in countercurrent ow with upwardly rising vapors. A liquid methane product leaves the tower through a pipe 30 and is conducted thereby to the heat exchanger 20 at a temperature of 260 F. and pressure of v50 p. s. i. a. for heatv exchange relation with the fluid being delivered to the fractionating tower through the pipe |9 to assist in obtaining the ultimate temperature of the fluid delivered to the fractionator, and effect evaporation of the liquid methane product.

From the heat exchanger 20 the cold methane is delivered through a pipe 3| to the passageway to further assist in maintaining the desired temperature of the gas delivered to the separator l5. The methane is discharged as a. gas from the heat exchanger through a pipe 32 that connects with a compressor 33 which raises the pressure for mixture with the gas discharged from the other compressor units to complete make-up of the product gas.

If low pressure product gas is desired the compressors, of course, may be omitted from the system since their only purpose is to impart the desired delivery pressure of product gas.

The nitrogen separated in the tower 25 leaves the top of the tower through a pipe 34 and passes through the heat exchanger 22 in heat exchange relation with the incoming liquid to the frac- After. passing through the heat and the temperature may be further reduced by passing it through an expansion engine 35 whereby it is expanded down to assume a temperature of 305 F. and then passed through the heat exchanger 2| where it gives up its cold to the incoming liquid, after which it is discharged through a pipe 36 into the heat exchange passageway 9 of the heat exchanger 5 so as to assist in maintaining the liquefaction temperatureof the incoming gas traveling through the passageway I0. After absorbing heat within the heat exchanger, the nitrogen is discharged through a pipe 31 `as Waste gas. y

The form of the invention illustrated by the ilow diagram in Fig. 2 is designed to remove more or all of the nitrogen. In this system all of the gas after passing through the carbon dioxide and hydrogen sulfide removal and dehydrator units 31 and 38 respectively is passed through a pipe tower 52.

4' 39 to a heat exchanger 40 where the temperature is reduced to partial liquefaction point at the pressure of the gas and is passed to a separator 4| through a pipe 42 where the liquid and vapors separate.

The liquid phase is taken from the bottom of the separator 4|v through a pipe 43 having a throttle valve 44 therein and is passed to a second stage separator 45 wherein the pressure is held low enough so that the liquid leaving the bottom thereof by Way of a pipe 46 contains practically no nitrogen. The cold is recovered from the liquid by flowing it through a passageway 41 of the heat exchanger 40 and is discharged therefrom through a pipe 48 as a part of the product gas.

The.vapor phase in the separator 4| which contains the nitrogen leaves the top of the separator through a pipe 49 and is conducted through a heat exchanger 50 and then through a reboiler 5| located in the base of the fractionating column or tower 52 and is therein' partially condensed. The partially condensed ow is then conducted through a pipel 53 into a separator l54 wherein the liquid portion of the flow drops to the bottom and is removed through a pipe 55 and introduced into the mid portion of the fractionating The pipe 55 is provided with a pressure reducing valve 6| so that the flow therethrough may be admitted at a pressure corresponding with the pressure,` in the fractionator 52. The vapor portion is removed from the top of the separator 54 through a pipe 56 and conducted through a reboiler 51 for total condensation. The

condensate is conducted from the reboiler 51 by a pipe 58 through heat exchangers 59 and 60 for subcooling and then ow is throttled by a valve 6| and discharged into the upper end of the fractionating tower 52 for counterow with gases rising in the tower.

Separated gas is removed from the top of the separator 45 through a pipe 62 and is conducted through a reboiler 63 in fractionating tower 52 for partial condensation and the partially condensed flow continues through a pipe 64 into a separator 65. The liquid component separates and is removed from the bottom through a pipe 66 and discharged at a mid point of the fractionating column. The nitrogen released in the fractionating tower 52 isY carried off through a pipe 61 for use as a cooling medium in the heat exchangers 59 and 60. By expanding the nitrogen in an expansion engine 68, located intermediate the heat exchangers59 and 60, the temperature of the coolant is further reduced to maintain lower temperatures in the exchanger '59 required to maintain liquefaction temperature of the ow into the top of the fractionating tower. The nitrogen still retaining substantial refrigeration is passed through a passage 69 of the heat exchanger40 and removed through a pipe 10 to be discharged as waste gas. The cold methane vapor is discharged from the bottom of the fractionating tower 52 through a pipe 1| that leads the cold vapor to the heat exchanger 50 for exchange with the gas conducted through the pipe 49. Continuing from the heat exchanger 50, the fluid is united with the ow in the pipe 46 and flows therewith through the passage 41 and pipe 48 to complete make-up of the product gas.

lf desired, the vapor may be removed from the separator 65 and discharged through the pipe 12 for use as fuel.

From the foregoing it is obvious that we have provided an economic method for removing nitrogen from natural gas by low temperature liquefaction and fractionation, the low temperatures necessary for liquefaction being maintained through heat exchange of the natural gas with the processed fractions.

v What we claim and desire to secure by Letters Patent is:

1. Themethod of treating natural gas influent for producing a product gas having a higher B. t. u. value including cooling the natural gas for effecting partial liquefaction of the natural gas, separating the gaseous portion from the liquefied portion, partially condensing the gaseous portion in a reboiler, separating the condensate from the vapors, passing the vapors through a second reboiler to effect condensation of said vapors. subcooling said condensate, fractionating the subcooled condensate to separate nitrogen and a product gas having a high B. t. u. value, mixing the product gas with the first separated liquefied portion, vaporizing the mixture by passing said mixture in heat exchange relation with the incoming natural gas to assist in maintaining said liquefaction temperature of the incoming natural gas, and passing the nitrogen from the fractionating zone in heat exchange relation with the incoming natural gas.

2. The method of treating natural gas for producing a product gas having a higher B. t. u. value including cooling an incoming stream of natural gas for effecting partial liquefaction of said stream, subcooling the portion of the stream remaining in gaseous state, passing the subcooled portion through a reboiler stage to effect partial condensation, passing the uncondensed portion from the reboiler through a second reboiler stage to effect substantially complete condensation, subcooling said last named condensate, fractionating all of said condensate into anltrogenous gas fraction and a gaseous fraction having a high B. t. u. value, expanding and performing work with the nitrogenous fraction for reducing temperature of the nitrogenous fraction to that required for subcooling said condensate, passing the cooled nitrogenous fraction in heat exchange relation with said condensate for maintaining said subcooling temperature of said condensate. passing the nitrogenous gas fraction from said heat exchange relation to heat exchange relation with the incoming natural gas for assisting in maintaining said partial liquefaction temperature, passing the liquefied portion of the natural gas in heat exchange relation with the stream remaining in gaseous state, mixing the liquefied portion of the natural gas with the portion liquefied in said first step, and passing said mixture and said gaseous fraction separately in heat exchange relation with the stream of incoming natural gas to assist in maintaining said partial liquefaction temperature and for vaporizing said liquefied portion.

3. The method of treating a natural gas influent for producing a product gas having a higher B. t. u. value including cooling the natural gas for effecting partial liquefaction of the natural gas, separating the gaseous portion from the liquefied portion, partially condensing the gaseous portion in a reboiler, separating the contionating the subcooled condensate to separate nitrogen and a gaseous fraction having a high B. t. u. value, uniting the fraction high in B. t. u. value with said previously liquefied portion of the 4. 'Ihe method of treating a natural gas influent for producing a product gas having a higher B. t. u. Value including cooling the natural gas for effecting partial liquefaction of the natural gas, separating the gaseous portion from the liquefied portion, partially condensing the gaseous portion in a reboiler, separating the condensate from the vapors, passing the vapors through a second reboiler to effect condensation of said vapors, subcooling said last named condensate, throttling flow of the liquefied portion, separating vapors from the liquefied portion, passing said vapors through a third reboiler for partial condensation, discharging the vapors from said third reboiler, fractionating the subcooled condensate together with the other condensate to separate nitrogen and a fraction having a high B. t. u. value, uniting said previously liquefied portion of the natural gas with the fractionhigh in B.A t. u. value and vaporizing the mixture to make up the product gas.

5. 'Ihe method of treating natural gas for producing a product gas having a higher B. t. u. value including cooling an incoming stream of natural gas for effecting partial liquefaction of said stream, again cooling the portion of the stream remaining in gaseous state, passing the cooled portion through a reboiler stage to effect partial condensation, passing the uncondensed portion from the reboiler through a second reboiler stage to effect substantially complete condensation, subcooling said last named condensate, throttling flow of Asaid liquefied portion, separating vapors from'the throttled liquefied portion, passing saidvapors from the throttled liquefied portion through a reboiler stage to effect partial condensation, discharging the vapors from the condensate resulting in said last named reboiler stage, fractionati-ng all of said condensate into a nitrogenous fraction and a gaseous fraction having a high B. t. u. value, expanding and performing work with the nitrogenous fraction for reducing temperature of the nitrogenous fraction to that required for said subcooling temperature, passing the cooled nitrogenous fraction in heat exchange relation with said condensate which is sub-cooled for maintaining said subcooling temperature, passing the nitrogenous gas fraction from said heat exchange relation to heat exchange relation with the incoming natural gas for assisting in maintaining said partial liquefaction temperature, mixing the liquefied portion of the natural gas and said gaseous fraction, and passing said mixture in heat exchange relation with the stream of incoming natural gas to assist further in maintaining said partial liquefaction temperature and for Vaporizing said mixture to make up the product gas.

WILLIAM W. BODLE. WALTER W. DESCHNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. THE METHOD OF TREATING NATURAL GAS INFLUENT FOR PRODUCING A PRODUCT GAS HAVING A HIGHER B. T. U. VALUE INCLUDING COOLING THE NATURAL GAS FOR EFFECTING PARTIAL LIQUEFACTION OF THE NATURAL GAS, SEPARATING THE GASEOUS PORTION FROM THE LIQUEFIED PORTION, PARTIALLY CONDENSING THE GASEOUS PORTION IN A REBOILER, SEPARATING THE CONDENSATE FROM THE VAPORS, PASSING THE VPAORS THROUGH A SECOND REBOILER TO EFFECT CONDENSATION OF SAID VAPORS, SUBCOOLING SAID CONDENSATE, FRACTIONATING THE SUBCOOLED CONDENSATE TO SEPARATE NITROGEN 